$1 represents first part within the () (i.e) src= and $2 represents the second part within the () (i.e) &, so this indicates you are going to change the value between src and &. More clear, it should be like this:

If you look closely you'll see two surprising things about URLs: (1) they seem simple, but the details and corner cases are actually hard, (2) Amazingly JavaScript doesn't provide a full API for making working with them any easier. I think a full-fledged library is in order to avoid people re-inventing the wheel themselves or copying some random dude's clever, but likely buggy regex code snippet. Maybe try URI.js (http://medialize.github.io/URI.js/)